stepi.f

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      REAL*8 FUNCTION stepi(F,FDOT,F2DOT,F3DOT,ETA)
*
*
*       Fast time-step expression.
*       --------------------------
*
      IMPLICIT REAL*8  (a-h,o-z)
      REAL*8  f(3),fdot(3),f2dot(3),f3dot(3)
*
*
*       Set square FDOT & F2DOT and sum of all absolute values.
      fd2 = fdot(1)**2 + fdot(2)**2 + fdot(3)**2
      f2d2 = f2dot(1)**2 + f2dot(2)**2 + f2dot(3)**2
      fi = abs(f(1)) + abs(f(2)) + abs(f(3))
      fd = abs(fdot(1)) + abs(fdot(2)) + abs(fdot(3))
      f2d = abs(f2dot(1)) + abs(f2dot(2)) + abs(f2dot(3))
      f3d = abs(f3dot(1)) + abs(f3dot(2)) + abs(f3dot(3))
*
*       Obtain time-step by simplified relative criterion.           
      stepi = eta*(2.0*fi*f2d + fd2)/(2.0*fd*f3d + f2d2)
      stepi = sqrt(stepi)
*
      RETURN
*
      END